1. Field of the Invention
The present invention relates to an optical radiation device, an image reader, and an image forming device used in a copier, a facsimile machine, or an image scanner.
2. Description of the Related Art
According to a related art image reader such as an image scanner, light is projected from a light source of an optical radiation device to a document on a contact glass, and the light reflected by the document is imaged on an image sensor such as a charge coupled device (CCD) via an imaging lens to read an image on the document. The light source of such an image reader is a stick-like light source as fluorescent lamp or xenon lamp or a point light source as LED. In particular, the LED is generally used in replace of a xenon lamp, aiming for increasing a rising speed and longevity, and saving energy.
Referring to FIG. 10, the effective read area of an image reader is described. A scale 002 is provided on a contact glass 001 to help users know a document set position and a paper size. Thereby, users can prevent a document 003 from being displaced on the contact glass and acquire a good image.
However, even by use of the scale 002, manually placing the document 003 may cause a slight displacement of the document. To be able to read a displaced paper, an effective read area is generally set to be wider than the document 003. For instance, in reading A4 size papers, the effective read area L is set to 219 mm, wider by 3 mm than the maximum document length of 216 mm. In FIG. 10 the document 003 is 216 mm in length.
The reading center 004 is set at the center of the document accurately placed in FIG. 10. This is to prevent a degradation of image reading quality which occurs because the illuminance on the light receiving surface declines as a portion of the document goes far from the reading center 004 due to a decrease in the illuminance of the optical collector of the image reader by the fourth power of cosine law.
The scale 002 is closely attached to the contact glass 001 and the document 003 is not displaced toward the scale 002. Because of this, the margin 3 mm of the effective read area L is on the opposite side to the scale 002 and the effective read area L can be divided from the reading center 004 to an area A on the scale side and an area B on the opposite side. When the center 004 is 0 mm and the scale side is negative and the opposite side is positive, the effective read area is from −108 mm to +111 mm and asymmetric relative to the center 004.
Next, a related art image reader is described with reference to FIG. 11. The image reader includes an LED 122 attached to a bracket 121, a first scanner 103 to which the bracket 121 with a V-shape cross section is attached, a substrate 123 attached to the bracket 121 to drive the LED 122, and a reflector 118 attached to the first scanner to reflect the light from the LED 122 to properly adjust illuminance distribution and eliminate shadows occurring in reading a document including a cut and paste portion.
In the image reader using the above scale-down optical system, it is a long distance between the surface of a document and the image sensor so that the light from the light source is largely attenuated. Therefore, the illuminance of the LED 122 of the image reader needs to be heightened. For this purpose, a number of LEDs 122 are arranged linearly in the main scan direction of the document.
Further, it is preferable to dispose the LEDs arranged in array on the bracket 121 obliquely to face the document on the contact glass 001 for the purpose of realizing good illuminance distribution in the document in sub scan direction. The illuminance distribution is preferably such that only the irradiation area E of the document surface or the actual read area of the document is radiated with light.
However, even if the light from the tilted LED 122 is reflected by the reflector 118 to the document, the areas other than the irradiation area E is also radiated with the light. In reading an image with a solid black portion between white portions, for example, the light reflected by the white portions in the irradiation area E enters the image sensor, increasing the output values of the solid black portion. Accordingly, the solid black portion cannot be reproduced accurately.
In solving such a problem, Japanese Patent Application Publication No. 2007-5860 (Reference 1) and No. 2010-130056 (Reference 2) disclose an optical radiation device with an optical guide extending over the exit surface of an LED array in main scan direction to project the light to the irradiation area with uniform illuminance distribution by guiding the light from the LED array thereto.
Further, it is also possible to achieve an arbitrary illuminance distribution in an image in main scan direction by adjusting the spacing between neighboring LEDs. This can be used to effectively correct a decline in illuminance of the optical collector by the fourth power of cosine law by arranging the LEDs with different spacings so that the further from the center the LEDs are, the smaller the spacing is, with the reading center of the optical collector matching with about the center of the optical guide. Also, the arrangement of the LEDs are symmetric relative to the reading center of the optical collector of the image reader.
The illuminance at the ends of the document surface is generally lower than that at the center since no LEDs are arranged outside the ends and it is affected by the frame supporting the contact glass. The decline in illuminance at the ends of the document surface can be prevented by the total reflection of the end surface of the optical guide.
Meanwhile, the optical guide is often made from an engineering plastic such as a transparent resin as PMMA (polymethyl methacrylate) or COP (cyclo-olefin polymer) by injection molding. An optical guide of a complex shape can be manufactured at a low cost by injection molding.
To form the optical guide by injection molding, it is necessary to provide a gate in the optical guide to introduce a resin thereinto. The position of the gate is decided depending on the shape of the optical guide. For a long optical guide, the gate is preferably provided at an end surface or side surface in the resin flow direction or the length direction. Thereby, the resin can flow smoothly in the length direction so that a molding failure can be prevented.
However, provided with the gate, the optical guide cannot exert total reflection at the end surface. In general the gate is formed by cutting the end of the optical guide with a cutter, therefore, the cut surface is rough and surface accuracy is not sufficient for the total reflection. The illuminance of the document surface from the end surface having the gate is lower than that from the other end due to a loss of light beams, which degrades image reading quality.
Reference 2 teaches increasing light amount from the end portion of the optical guide using total reflection, however, it does not consider or concern the decline in illuminance due to the gate. To acquire the surface accuracy of the end portion for the total reflection after the gate is formed by cutting, secondary processing such as wrapping is required, which takes additional time and costs and increases the total manufacture costs of the image reader.